Wideband Code Division Multiple Access (WCDMA) telecommunication systems have many attractive properties that can be used for future development of telecommunication services. In order to retain stability of a cell, the load needs to be kept below a certain level. This follows since the majority of uplink user channels, at least in WCDMA, are subject to power control. This power control aims at keeping the received power level of each channel at a certain signal to interference ratio (SIR), in order to be able to meet specific service requirements.
Since the Radio Base Station (RBS) tries to keep each channel at its specific preferred SIR value, it may happen that an additional user, or bursty data traffic of an existing user, raises the interference level, thereby momentarily reducing the SIR for the other users. The response of the RBS is to command a power increase to all other users, something that increases the interference even more. Normally this process remains stable below a certain load level. In case a high capacity channel would suddenly appear, the rise in the interference becomes large and the risk for instability, a so called power rush, increases. It is thus a necessity to schedule high capacity uplink channels, like the enhanced uplink (EUL) channel in WCDMA, so that one can insure that instability is avoided. In order to do so, the momentary load must be estimated in the RBS or any node connected thereto. This enables the assessment of the capacity margin that is left to the instability point.
One approach to improve load estimation is disclosed in the published international patent application WO 2006/076969. A minimum value of a power quantity, preferably a difference between the instantaneous total received wideband power and the instantaneous sum of powers of all links used in the same cell, is used as an estimate of an upper limit of the thermal noise floor, based on which a noise rise measure can be estimated. An optimal and soft algorithm for noise rise estimation based on a similar basic idea of minimum values is disclosed in the published international patent application WO 2007/024166.
To handle increasing uplink data rates, interference cancellation (IC) or interference suppression (IS) is being introduced in WCDMA. A conventional procedure to perform IC is summarized by the following steps. A channel model of the interferer to be cancelled is estimated. This does not cause any additional operations, since this channel model is anyway needed. The transmitted signal of the interferer to be cancelled is also decoded. This is also anyway needed. A replica of the received signal of the interferer to be cancelled is then created, by use of the channel model and the decoded signal. This replica may e.g. be reconstructed as an IQ chip stream. The replica of the interfering signal is subsequently subtracted from the received signal of the user to be decoded, thereby hopefully reducing the remaining power of the interferer to very low power levels.
This procedure obviously affects the load measurement functionality of the WCDMA EUL. Thus, the scheduler must be aware of the instantaneous uplink load in several signal points of any IC receiver structure in order to be able to utilize the entire amount of resources. Unless such load estimates are made available it will not be possible to exploit the link gains fully, when scheduling EUL users. In the published international patent application WO 2008/097145, load estimation with IC of this conventional type is handled in analogy with earlier noise rise estimation procedures.
Another approach to limit the effect of interference is to use some kind of interference whitening approaches. Minimum mean square error (MMSE) Frequency Domain Equalization (FDE) is a common technique to combat frequency selectivity in wideband channels, like in the WCDMA uplink. The interference whitening is performed for each user separately. The main advantage associated with FDE is a relatively low complexity. The advantages come with a price due to the fact that linear convolutions are replaced by circular convolutions when going to the frequency domain.
In order to utilize the reduced interference provided by the FDE, the load estimations have to be performed taking the interference whitening of the FDE into account. However, in contrary to what is valid for interference cancellation based on regeneration and subtraction, the thermal noise floor is changed in the same manner as the interference reduction obtained by the FDE process, and can no longer be treated as constant after interference whitening. A treatment analogue of WO 2008/097145 can therefore not be used for achieving a noise rise estimation, the reason being that WO 2008/097145 requires the noise floor to be constant. There is thus a problem of using the created reduced interference, since no reliable noise rise estimation is available.